1. Field
One or more aspects of embodiments according to the present invention relate to a DC-DC converter and a method of manufacturing the same.
2. Description of Related Art
DC-DC converters are used to convert between higher and lower DC voltages in a variety of different devices. While some devices include one or only a few DC-DC converters, some systems include many hundreds to thousands of DC-DC converters. For systems that include hundreds or thousands of DC-DC converters, DC-DC converters can take up a lot of space. Further, as each DC-DC converter wastes power through heat loss, hundreds or thousands of DC-DC converters can result in loss of a large amount of energy.
A common DC-DC converter that takes as input a higher DC voltage, and converts it to a lower DC voltage, is known as a buck converter. A buck converter is typically more efficient than a linear power regulator. A typical linear voltage regulator may dissipate (Vin-Vout)*Iout watts as wasted heat, wherein Vin is the input voltage, Vout is the output voltage, and lout is the current output. For example, a 40V to 28V linear regulator delivering 1 amp to a load may dissipate (40-28)*1=12 watts (W) as heat, while delivering only 28V*1 amp=28W to the load. The efficiency is therefore Pout/(Pout+Ploss)=28/(28+12)=28/40 =only 70%.
A buck converter can convert 40V to 28V at over 93% efficiency, with only 2 watts of power (heat) loss. For example, for a typical buck converter, Pout/(Pout+Ploss)=28/(28+2)=28/30=93%. A buck converter is a member of the switching mode power supply (SMPS) family, and hence uses a switching frequency, as well as a transistor switch, a diode, an inductor, and a capacitor.
A typical buck converter is illustrated in FIG. 1, and includes a FET M1 that receives a pulse width modulation (PWM) signal input at its gate from a PWM circuit 20. The buck converter of FIG. 1 also includes an inductor L having a first end coupled to a source of the FET M1. A drain of the FET M1 is coupled to the positive terminal of a DC power source 10 having voltage Vs. A diode D is coupled between the first end of the inductor L and a negative terminal of the DC power source 10. A capacitor C and a resistor RL are coupled in parallel between a second end of the inductor L and the negative terminal of the power source 10.
In some cases, the diode D is used together with a second transistor in a “synchronous” buck converter, which is shown in FIG. 2. The synchronous buck converter of FIG. 2 is substantially similar to the buck converter of FIG. 1, except that a second FET M2 is coupled across the diode D in parallel. For example, as can be seen in FIG. 2, a drain of the FET M2 is coupled to a cathode of the diode D and a source of the FET M2 is coupled to an anode of the diode D. The PWM output signal from the PWM 20 is also provided to a gate electrode of the FET M2 in addition to the gate electrode of the first transistor FET M1. For example, the diode D may only conduct when the FET M2 is off, and the output voltage would vary depending on the duty cycle of the PWM output signal provided to the FET M2. By way of example, the PWM control signals to the FETs M1 and M2 may have different phases and duty cycles.
A synchronous buck converter may be more efficient that a typical buck converter because the power (heat) loss through a field effect transistor (FET) is I^2*Ron, whereas the power loss thru a diode is Ploss=Vfwd*I, where I is the load current, Ron is the FET on-resistance, Vfwd is the forward voltage drop of the diode, and Ploss is the power loss. Since a FET can be designed with low on resistance, whereas a diode forward voltage drop is fixed by the diode material band gap, the FET can be made to have lower loss.